Gel electrophoresis is one of the most convenient and widely used methods for separating proteins and nucleic acid molecules. The macromolecules separated on the gel are generally unaccessible to various probes which are required for analysis, and it is accordingly necessary to transfer the molecules from the gel to a suitable filter sheet so as to enable the probes' access to the separated molecules. The molecules adsorb to the filter sheet to which they immobilize while maintaining their respective position as in the gel--a process known as "blotting".
Suitable filter sheets used for blotting include nitrocellulose membranes, nylon filters, diazo-modified papers, cyanogen-bromide activated paper and ion-exchange paper. The transfer of the separated molecules to the filter sheet can be carried out by vacuum blotting, pressure blotting, electroblotting and capillary blotting. DNA molecules are usually blotted by means of pressure or vacuum, while protein molecules can also be blotted by the application of an electric field (electroblotting). (Gershoni J. M. et al., Protein Blotting: Principles and Applications, Analytical Biochemistry, 131, 1-15 (1983)).
After they are transferred to the filter sheet the macromolecules become accessible to probes, such as antibodies, nucleic acid probes and the like, and the identity of the macromolecule can thus be revealed. However, in order to recover macromolecules of interest from the filter sheet, the portion of the sheet containing the respective macromolecules must be cut from the sheet and the desired molecules are then eluted by suitable reagents. This recovery process presents several problems. First, it is not always simple to determine exactly the boundaries between different macromolecules on the sheet. Second, the use of an eluent necessarily dilutes the molecules.
The direct recovery of macromolecules from the gel by electroelution, gel dissolution, gel compression or diffusion also presents several problems. (Hamilton O. Smith, Recovery of DNA front Gels. Methods in Enzymology, Vol. 65, 371-380 (1980)). Recovery of micrograms or nanograms of DNA or protein from a gel weighing several milligrams is inherently a very difficult task and accordingly, all these procedures are very time and labor consuming. This problem is especially pronounced where a number of DNA or protein fractions have to be purified from a single sample.
The nature of the filter sheets currently used for blotting present several further problems. Nitrocellulose membranes have a relatively low binding capacity in the order of 80-100 .mu.g protein/cm.sup.2, which is a drawback where multiple probing is desired, as proteins are lost during washes between the contacts with the different probes and thus only one or very few washing and probing cycles are possible.
Nylon membranes generally have a higher capacity for proteins. For example, ZETABIND.TM. (AMF Cuno Division, USA) or ZETAPROBE.TM. (Bio-Rad Laboratories, USA) which is a nylon matrix which has been modified by extensive cationization, has a binding capacity of 480 .mu.g protein/cm.sup.2. This high binding capacity is also a problem, since areas of the nylon filter not occupied by the separated proteins can adsorb the probes non-specifically during the incubation period, which results in an intolerably high background signal. This may, at times, be avoided by saturating the filter's unbound sites by proteins such as BSA and hemoglobin.
Other filter sheets used such as diazo-modified papers (diazo-benzyl-oxymethyl paper, diazo-phenyl-thio-ether paper) cyanogen bromide activated paper and ion-exchange papers have a relatively low protein binding capacity in the order of 25-50 .mu.g protein/cm.sup.2. These filter sheets need to be activated prior to use and have in general a low resolution, which properties render them often unsuitable.
Magnetic particles are used for a variety of techniques based on affinity binding of molecules to the particles and then isolating the particles by application of a magnetic field. Such magnetic separations have been employed to sort cells, to recover antibodies or enzymes from solutions, to purify proteins using affinity techniques and to remove unwanted particles from suspensions (Pourfarzaneh M. K. et al., The Use of Magnetizable Particles in Solid Phase Immunoassay in Methods of Biochemical Analysis, Vol. 28, pp 267-275 (1982)).